NITIN KUMAR - Thermal Food Engineering Operations

Sometimes when microorganisms survive any environmental stress, they might again revive and regrow themselves with available appropriate conditions [35]. This shortcoming can cause the wrong estimation of the exact lethality after treatment as it might not detect properly only the cells which are injured. There is a chance therefore to get repair during the treatment and reviving phase. So, this can be avoided by adding some further agents for presentation so the reviving process of the cell can be containing and better inactivation can be achieved [36]. Hence, uncovering and classifying the sublethal injury by innovative preservation techniques is indispensable for optimizing a varied combination of methods for elevated effects of microbes for inactivation.

As already discussed, microbes tend to grow resistant for some time when coming in contact with varied environmental stress which causes serious issues when it comes to food safety.

Researchers have been studying for many years now how to deal with varied adaptation techniques to do the inactivation process better. Modifying the sigma factor with different RNA polymerase is probably the most significant controlling method in bacterial cells [37]. This factor governs the transcription of the genes in Gram-negative bacteria which are resistant to oxidative, heat, and osmotic stress. Therefore, inducing these sigma factors would help to activate during the cell undergoing different growth phases, may it be in stationary or exponential state [38]. But for Gram-positive bacteria a substitute sigma factor with alike physiological roles are studied in [39, 40]. This infers that a similar process for multiple stress resistance is seen for cells of Gram-negative and Gram-positive. The utmost problem that should be taken care of is to prevent microorganisms from adapting to the stress because that helps them to create a barrier and create greater protection for different other succeeding stress. This is the reason for the emergence of different novel techniques for preservation; a direct heat and traditional thermal method is causing sublethal injury as well as augmenting the sensitivity of the cells to stress when applied mechanically. This is because of some temperature-induced variations in the cell envelopes of microorganisms [41].

The main cause of the oxidative stress in bacteria is due to the following reasons, i.e., imbalance of macromolecules changes, cellular and intracellular antioxidant and oxidant concentration which is related to lipids, proteins, and DNA repair enzymes [42]. Enzymes are considered to be the shield of microbes and catalyses Hydrogen peroxide which is the prevailing bactericide (also for spores) and oxidant that is capable of generating chemicals that can oxidize hydroxyl radicals (OH˙) [43]. Peroxidases convert to alcohol and water by reducing hydrogen or organic peroxides.

The requirement of water in the food system varies and is calculated through the water activity (aw). The addition of solutes in the microbial cells changes the water activity which results in lowering the water content, causing osmotic stress. And due to this, microbes increase their cytoplasmic cells through various processes, for instance, humidity, air circulation, limitation of nutrition, and tempura which contribute to the stress [42].

Elevated pressure when applied to the microbes results in altering the genes, metabolisms, and morphology of the cell. It is known that microorganisms adapt easily to the environment using various methods. They utilize diverse protection processes by activating the expression of the genes, staying in the dormant state, and producing the resistant mutant. There are different ways microorganisms adapt to the adverse condition, such as they create spores which hardly changes its morphology during application of pressure. As mentioned above, the resistance power to pressure in the stationary phase higher as compared to the exponential phase [44]. As the structure of the cell does not change in the stationary phase due to the protection of the membrane, the tolerance stress level gets elevated [45]. Listeria innocua endured in replicated milk with supplementary magnesium, calcium, citrate, and phosphate [27]. Magnesium has the stabilizing agent for ribosomes and calcium aids in maintaining the outer wall of the membrane of the cell. Sucrose guards bacteria by stratifying the functionality of membrane proteins [46].

Infrared falls in the electromagnetic spectrum fluctuating between 0.78-1000 μm which is between ultraviolet and microwave radiation. Heat is generated due to the motion of the molecules which is both rational and vibrational in nature. Unlike conventional heating where the heating is done by convection at the first from the surface and from inside of the product, it is done by conduction, here infrared offers radiation from outside the surface and conduction from inside of the food product [47]. Numerous studies have been done to verify the anti-microbial effect of IR when applied to various food products such as honey, cheese, milk, fruits, and other liquid and semiliquid products [48, 49]. Infrared works well even for powder products mainly spices and with time varied of the word have also been performed to analyze the effectiveness of the IR treatment for effectiveness in reducing microbial activation. Several factors significantly affect the microbial log reduction but the utmost responsible factor is the temperature and wavelength of infrared radiation. Some of the other factors are as following power, water activity, moisture content, bandwidth, and depth of the food sample [50].

Infrared offers a similar effect of thermal inactivation mechanism as seen in the microwave heating and ultraviolet light which causes DNA damage along with heating through induction and of course. By thermal heating, the inactivation of microorganisms becomes easier; it can destroy or damage different parts of the cell structure which mention in the order of damage magnitude that is protein > RNA > cell wall > DNA. To confirm the effects and study the mechanism of heating for the inactivation of microbes, various methods like fluorescent and spectroscopic probes were utilized for paprika powder performed by [51]. The result inferred from the treatment that radiation of the infrared waves created all-around injury of the cell which includes inactivation of RNA polymerase of the microbial cells, which simultaneously prevents the transferase reaction by combining it with subunits of the ribosome. Much effective and advanced decontamination is visible in this treatment as the waves of the infrared have extended energy levels, and distribution of this energy is very efficient as compared to any other traditional method of heating which mostly uses the fluorescent probe for molecular-level analysis. Analysis using the digital method showed the best results as compared to the conventional one in terms of analyzing the relationship of the targeted organism and the environment they are present in real time.

Different studies were conducted which explain the different effect of the parameters of infrared radiation. Bacillus cereus on paprika powder was studied by applying radiation at 11 kW/m 2and 5 kW/m 2at a temperature of 95 °C [51]. From the studies it showed that maximum injury of microbes was seen at a w0.5 and at a w0.8 the overall log reduction was seen was 0.7 and 1.6 log 10 CFU/g at 5 and 11 kW/m 2. From this, it was inferred that Bacillus cereus is susceptible to heating through infrared, plus also preserving the effects of the main product. A similar effect was studied for oregano powder where disinfection of the microbes was done for investigation [52].